Calculation Procedure:- Place the unit load in position, and compute the
bending moment
The moment of all forces acting on the truss at panel points to the left of b with respect to
b is termed the bending moment at that point. Assume that the load transmitted to the giv-
en truss is 1 kip (4.45 kN), and let x denote the instantaneous distance from the right-hand
support to the moving load.
Place the unit load to the right of C, and compute the bending moment Mb. Thus RL =
/120; Mb = 45RL = 3/8, Eq. a. - Place the unit load on the other side and compute the
bending moment
Placing the unit load to the left of B and computing Mb, Mb = 45RL - (x - 75) = -5*/8 +
75, Eq. b. - Place the unit load within the panel; compute the panel-point
load and bending moment
Place the unit load within panel BC. Determine the panel-point load PB and compute Mb.
Thus - PB (x - 60)/30 = x/30 - 2; Mb = 45RL -15PB = 3x/8 - 15(x/30 - 2) = -jc/8 + 30, Eq. c. - Applying the foregoing equations, draw the influence line
Figure 506 shows the influence line for Mb. Computing the significant values yields CG =
(3/8)(60) = 22.50 ft-kips (30.51 kN-m); BH = -(5/8)(90) + 75 = 18.75 ft-kips (25.425
kN-m). - Compute the slope of each segment of the influence line
This computation is made for subsequent reference. Thus, line a, dM^dx = 3/8; line b,
dMddx = -5/8; line c, dMJdx = -1/8.
FORCE IN TRUSS CHORD CAUSED BY
MOVING CONCENTRATED LOADSThe truss in Fig. 5Oa carries the moving-load system shown in Fig. 51. Determine the
maximum force induced in member BC during transit of the loads.
Calculation Procedure:- Assume that locomotion proceeds
from right to left, and compute the
bending moment
The force in BC is a function of the bending moment
Mb at b. Refer to the previous calculation procedure
for the slope of each segment of the influence line.
Study of these slopes shows that Mb increases as the
load system moves until the rear load is at C, the
front load being 14 ft (4.3 m) to the left of C. Calcu-
late the value of Mb corresponding to this load dispo-
sition by applying the computed properties of the in-
fluence line. Thus, Mb = 22.50(24) + (22.50 - 1/8 *
14)(6) = 664.5 ft-kips (901.06 kN-m). FIGURE 51